The present invention relates to a catalytic process for preparing cinnamates by oxidative carbonylation of styrene compounds.
Cinnamic acid and cinnamates are used as a material for perfumes, as a cinnamic aldehyde, cyclamen aldehyde, beta-amyl cinnamic aldehyde, and the like.
Cinnamates are made conventionally through a Claisen condensation from benzaldehyde and alkylacetate in the presence of sodium alkoxide, as shown in Equation I EQU C.sub.6 H.sub.5 CHO+CH.sub.2 COOR.fwdarw.C.sub.6 H.sub.5 CH.dbd.CHCOOR+H.sub.2 O (I)
Another method of making cinnamates is by esterification of cinnamic acid, as shown in Equation II EQU C.sub.6 H.sub.5 CH.dbd.CHOOH+ROH.fwdarw.C.sub.6 H.sub.5 CH.dbd.CHCOOR+H.sub.2 (II)
Recently, several methods for preparing cinnamates have been reported, employing palladium catalysts. Heck et al., J. Amer. Chem. Soc. 91, 6707 (1969) and Patel et al., J. Org. Chem., 42, 3903 (1977), show methods of preparing cinnamates using palladium acetate-tertiary phosphine as a catalyst in the reaction of phenyl bromide and an alkyl acrylate. This reaction has the drawback of involving rather expensive raw materials. This reaction is shown in Equation III. ##STR1##
Other cinnamates, such as methyl cinnamate, can be synthesized in a palladium catalyzed reaction by reacting styrene with carbon monoxide and methanol, as reported by J. K. Stille and his coworkers, J. Amer. Chem. Soc., 98, 1806 (1976) and 98, 1810 (1976); J. Org. Chem., 44, 3474 (1979); and by G. Cometti and G. P. Chiusoli in J. Organometal. Chem., 181, C14 (1979).
In Stille's method, methyl cinnamate could be obtained only in a small amount, with dimethyl phenylsuccinate being the major product. Additionally, a stoichiometric amount of copper (II) salt was required in this reaction. The method of Cometti and Chiusole has the same disadvantage of using a large excess of copper (II) salt as an oxidant. Thus, both methods are unsuitable for industrial applications.
Many patents disclose oxidative carbonylation of olefins to alpha, beta-unsaturated esters by reacting an olefin with carbon monoxide, oxygen, and an alcohol in the presence of a catalytic amount of palladium and copper salts, cf. U.S. Pat. Nos. 3,381,030; 3,397,225; 3,397,226; 3,530,168; 3,621,054. None of these patents discloses a satisfactory method for producing cinnamates.
Two Japanese patent applications, Nos. 21,342 (1982) and 21,343 (1982), disclose that low yields of methyl cinnamate could be achieved through oxidative carbonylation of styrenes, provided that an excess amount of dehydrating agent is used in the reaction. Because the dehydrating agent is an expensive component of the reaction, industrial application of this method is limited. It was disclosed in these two Japanese applications that when styrenes were allowed to react with aliphatic alcohols, carbon monoxide, and oxygen in the presence of palladium and a dehydrating agent, cinnamates could be obtained with a high reaction rate and a high yield. However, this reaction has the disadvantage of requiring an excess of dehydrating agent, which makes the reaction unfeasible for industrial use.